The long-haul optical transmission system realizes economical and large-volume information transmission by applying the WDM (Wavelength Division Multiplexing) transmission technology which multiplexes a plurality of optical signals with various wavelengths and transmits them through one optical fiber. In order to reduce a cost of the WDM transmission device, a transmission speed per one wavelength is upgraded to high speed. The transmission speed of 10 gigabits per second (Gbit/s) for each wavelength is put into practical use currently, and furthermore a transmission technology for 40 Gbit/s and 100 Gbit/s has been studied.
When the transmission rate is speeded up to 40 Gbit/s and 100 Gbit/s from 10 Gbit/s, it becomes a main problem to improve the optical noise tolerance, that is, SN ratio (Signal to Noise ratio). In other words, in the case of long-haul transmission, the transmission length is generally limited due to the optical noise arising in an optical amplifier which is used on a transmission line and in an optical transmitter and receiver. Therefore, if the same modulation and demodulation system as that for 10 Gbit/s transmission rate is used for 40 Gbit/s transmission rate, the noise tolerance is reduced to a quarter. For this reason, in the case of the 40 Gbit/s transmission rate, it is necessary to adopt a modulation and demodulation system with the strong optical noise tolerance. The configuration is currently a typical system where RZ-DPSK system or RZ-DQPSK system is applied as the modulation/demodulation system and a balanced receiver using a delayed interferometer is applied in the receiving side.
An example of the above-mentioned optical reception device is described in patent literature 1. FIG. 12 shows the configuration of the related optical reception device 600 described in the patent literature 1. The optical reception device 600, which demodulates the RZ-DPSK signal, includes a related optical receiver 610 and a related 1-bit delayed interferometer 650. The 1-bit delayed interferometer 650 is provided with a 1-bit delay element in one optical waveguide of a set of optical waveguides, and outputs a set of two optical signals 652 and 653 which correspond to a phase difference between adjacent bits of one optical input signal 651.
The optical receiver 610 includes two photodiodes (PD) 611 and 612 and a transimpedance amplifier 620. The photodiodes (PD) 611 and 612 convert two optical signals outputted from the 1-bit delayed interferometer 650 into intensity modulated signals. The transimpedance amplifier 620 is provided with a differential amplifier with a differential negative feedback 622 and is connected to the photodiodes (PD) 611 and 612. The transimpedance amplifier 620 obtains the intensity modulated signals from the photodiodes (PD) 611 and 612 and demodulates the RZ-DPSK signal through outputting the difference between them.
In the related optical reception device available for the RZ-DPSK modulation, it is necessary that the phase difference by 1 bit between two signals is accurately kept and that the intensities of the signals are equal until the demodulation is carried out. However, the received intensity of two optical signals may not be kept equal in some cases due to the difference in the intensity or optical path between two optical signals on the path through which the optical inputting signal passes through the 1-bit delayed interferometer and lenses and then is inputted into the photodiode. The difference in the received intensity of these signals degrades CMRR (Common Mode Rejection Ratio) and causes waveform distortion and an increase of jitter after the demodulation. Moreover, it is difficult to control the optical received intensity mentioned above with a high degree of accuracy.
A technology for solving those problems is described in the patent literature 1. As shown in FIG. 13, another related optical reception device 700 described in the patent literature 1 is provided with a related optical receiver 710 and the 1-bit delayed interferometer 650. The optical receiver 710 includes two photodiodes (PD) 711 and 712, a transimpedance amplifier 720 with a differential negative feedback, and a level adjustment unit 730. The transimpedance amplifier 720 is provided with a differential amplifier 721 with a differential negative feedback 722, and is connected to the photodiodes (PD) 711 and 712. The level adjustment unit 730 is connected to the transimpedance amplifier 720 and has a function of adjusting the levels of positive and complementary signals in two closed feedback loops. By adjusting the levels of positive and complementary signals in two closed feedback loops, the difference in the intensities between two signals before demodulation is corrected.
On the other hand, the coherent detection system is well known where the detection is performed by mixing a signal light with a reference light and detecting an interfering signal (beat signal) which is generated by the mixture. FIG. 14 shows an example of the configuration of a related coherent optical reception device which is applied to the coherent detection system. The related coherent optical reception device 5000 receives an optical reception signal 5001 and a local oscillation light 5002 whose wavelength is almost equal to the optical reception signal 5001 from a local oscillation light source, and makes the local oscillation light 5002 and the optical reception signal 5001 interfere each other, and converts the interference signal into an electric signal (coherent detection). Since the coherent detection system has strong dependency on polarization, one optical receiver receives only an optical signal whose polarization state is identical to that of the local oscillation light. Then, the related coherent optical reception device 5000 is provided with a polarization demultiplexing unit 5010 at the input part of the optical reception signal 5001. The polarization demultiplexing unit 5010 demultiplexes the optical reception signal 5001 into two orthogonal polarization components. As a result, although it is necessary to use two optical receivers in order to receive one optical signal, this disadvantage can be compensated by making an amount of transmission information two times larger using polarization multiplexing scheme.
Each polarization light of the optical reception signal 5001 and the local oscillation light 5002 are inputted into an optical 90 degrees hybrid circuit 5100. The optical 90 degrees hybrid circuit 5100 outputs four kinds of output light in total, that is, a pair of output light which are obtained by making each polarization light and the local oscillation light interfere in normal phase and reversed phase, and another pair of output light which are obtained by making each polarization light and the local oscillation light interfere in quadrature phase (90 degrees) and inverted quadrature phase (−90 degrees). These output optical signals are converted into current signals by two photodiodes 5200 for a pair of output light, and then are inputted into a differential trans impedance amplifier 5300. Since their direct current components are balanced (canceled) consequently, it is possible to extract efficiently only the beat components between the optical reception signal 5001 and the local oscillation light 5002. The electrical signals outputted from the differential transimpedance amplifier 5300 correspond to an in-phase component (I component) and a quadrature component (Q component) of the interference between the optical reception signal and the local oscillation light, respectively.
The output signals for every polarization, that is, four kinds of the electrical signals in total which are composed of the I component and the Q component of X polarization and the I component and the Q component of Y polarization, are converted very fast from analog signals to digital signals by an analog-to-digital conversion unit (ADC) 5400, respectively. The electrical signal is converted into the digital information signal and then is inputted into a digital signal processing unit (DSP) 5500. It becomes possible to carry out various equalization and decision processes on the above-mentioned digital signal by applying the digital signal processing technology which is widely used in the field of the wireless communication. After carrying out the digital signal processing and the error correction processing, the super high speed (for example, 100 Gbit/s) information signal is outputted. Patent Literature 1: WO 2009/069814 (FIG. 1 and FIG. 11)